The present invention relates to a method of and apparatus for rolling, by means of a full continuous hot strip mill or a planetary mill, a steel slab extracted from a heating furnace. In this specification, this steel slab will be referred to simply as "slab", hereinafter.
In the conventional hot strip mill, as well as in the planetary mill, the length of the slab is limited from 6 to 13 m, due to the size of the furnace and for the convenience sake in transportation and handling.
As a result of rolling of this slab by a hot strip mill, a so-called "fish tail" is formed at the trailing end of the slab after the coarse rolling. Therefore, usually, it is necessary to shear the leading and trailing ends of the slab by means of a crop shear disposed at the upstream side of the finishing rolling mill, before the finish rolling. The "fish tail" is also observed in the strip after a rolling by a planetary mill. The "fish tail" inconveniently causes a reduction in the yield of the material.
In the hot strip mill, when a succeeding slab is introduced into the finishing rolling mill after the preceding slab has passed the finishing rolling mill, the strip is inconveniently floated by the force of air as it passes the hot runner table, if the speed of the strip is high. It is, therefore, necessary to reduce the rolling speed down to the level of 60% of ordinary maximum rolling speed, in this transient period. The ordinary maximum rolling speed may be recovered only after the leading end of the new strip has reached a down coiler to permit the application of tension to the strip.
In case of the planetary strip mill, a small gap exists between the preceding and succeeding slabs, although the slabs are fed continuously. Therefore, as this gap passes the rolling mill, vibration and noise which is 1.5 to 2.0 times as high as those in the ordinary rolling are generated to shorten the life of the machine parts of the mill. In some cases, the slabs are ejected from the planetary mill with their trailing ends unrolled and still having a large thickness. If this slab having a large thickness is introduced into the subsequent finishing or planishing mill, an extraordinarily high rolling load will be caused in the latter. To avoid this, conventionally, it has been necessary to quickly open the rolls of the planishing mill by means of hydraulic cylinders, when the thick trailing end of the slab passes the planishing mill. This also results in a reduction of yield, because of the off-gauge of the thickness immediately before and after the trailing end of the slab having large thickness.
To sum up, the conventional rolling techniques involve the following problems.
(1) Fish tails are formed in the end of the rolled slab, due to the presence of free or unrestrained leading and trailing ends of the slab during rolling.
(2) In the hot strip mill, while the leading end of a slab is positioned between the finishing mill and the down coiler and, hence, unrestricted, the slab will be inconveniently floated by air if the rolling speed is high. Therefore, it is necessary that the rolling has to be made at a reduced speed until the leading end is caught by the down coiler.
(3) In the planetary mill, the rolling is rendered unstable at the gap between the preceding and succeeding slabs, to cause a vibration and noise.
In the field of rod rolling, although not directly related to the rolling of slabs, a method called "endless rolling" has been proposed and actually used. This method is to continuously roll the rod materials which have been welded previously in series, into continuous rod material. Typical examples of this method are disclosed in the Synopsis of 93rd symposium of Steel Association of Japan, held on 4th to 6th April, 1977 (Vol. 63, No. 4, 1977), and also in Japanese Patent Laid-open No. 135863/1978. In these known techniques, successive materials are welded over the entire areas of their abutting surfaces.
This known endless rolling method does not suggest at all the continuous rolling of slabs of the invention, because the application of welding of successive rod materials over the entire abutting surfaces to the rolling of slabs is quite inappropriate and disadvantageous in view of the rolling speed and the feeding speed of slabs, particularly in relation to the capacity of the welder.
Meanwhile, Japanese Patent Laid-open Publication No. 112459/76 discloses an endless rolling method in which the slabs are superposed and welded at their opposing ends. The length of the overlap of the slabs is a half or more of the slab width. It is considered that this endless welding of slabs will affect the rolls of the rolling mill more adversely than the existing rolling method, although it may be effective in eliminating the formation of the fish tail. Also, this continuous rolling of slabs can provide only a small yield of the product and, therefore, has only little practical utility as a method of continuous rolling of slabs.